1. Field of the Invention
The present invention relates to SCR (Selective Catalytic Reduction) systems in general and in particular to catalyst protection assemblies for such systems.
2. Description of the Related Art
Selective catalytic reduction systems catalytically reduce flue gas NO.sub.x from combustion systems such as power generation boilers to N.sub.2 and H.sub.2 O using ammonia in a chemical reduction process. This technology is the most effective method of reducing NO.sub.x emissions especially where high reduction percentages (70-90%) or low stack emission levels are required. NO.sub.x emissions from boilers are typically 90 to 95% NO with the balance being NO.sub.2. However, when the flue gas leaves the flue stack, the bulk of the NO is oxidized to NO.sub.2 which reacts in the environment to result in acid rain as well as producing smog constituents.
In boilers using SCR systems, the catalyst is housed in a reactor which is strategically located within the flue gas system. This location permits catalyst exposure to proper SCR reaction temperatures. The reactor design includes a sealing system to prevent flue gas bypassing and an internal support for structural stability of the catalyst. The reactor configuration can be vertical or horizontal depending on the fuel used, space available and upstream and downstream equipment arrangement. Uniform flow distribution of ammonia is required for optimum performance.
Ammonia is introduced upstream of the SCR reactor either in the form of anhydrous ammonia or vaporized aqueous ammonia.
Anhydrous ammonia can be introduced into the flue gas stream using relatively little energy. The pressurized anhydrous ammonia is evaporated with either small electric source or with steam coils. The ammonia vapor is then diluted with air to provide the mass necessary to distribute the reagent evenly over the ductwork cross section. The diluted ammonia-air mixture is delivered to a grid of injection pipes located in the flue gas ductwork. The major disadvantage with using anhydrous ammonia is the safety concerns associated with the handling and storage of pressurized anhydrous ammonia. In fact, many local regulations often require aqueous ammonia to be used instead of anhydrous ammonia. This is especially true in highly populated areas.
Aqueous ammonia is typically purchased in industrial grade approximately 30% by weight ammonia and 70% by weight water. A dedicated heater, usually of the electric type, is used to heat the dilution air to a level adequate enough to vaporize the required water and ammonia. A vaporization chamber or static mixer is used as the medium in which the phase change occurs. Usually atomization air is required to assist the break up of aqueous ammonia into fine droplets as it enters the vaporization chamber. The ammonia vapor-water vapor-air mixture exits the vaporization chamber and is delivered to an injection grid where injection occurs through a grid of injection pipes located in the flue gas ductwork upstream of the SCR catalyst bed.
It is well known that catalyst reactivity of the catalyst bed is adversely affected by contact of the catalyst with water, even small quantities of water. Therefore, condensation of water vapor onto the SCR catalyst surface results in premature deterioration of the catalyst. The process of condensation occurs if the temperature within the SCR system drops below the dewpoint temperature, which occurs when the boiler is off-line especially during boiler outages, as well as from moisture from boiler tube washing, air heater washing, and boiler tube leaks.
Known attempts at preventing such adverse condensation included the placing of tarps constructed of canvas, plastic, or other material over the catalyst during such outages or washing conditions. This provided incomplete protection of the catalyst and required extensive maintenance to place and fasten the tarps onto the catalyst. Personnel safety risks were also associated with this method from having the personnel work in a confined space.
Other systems established a constant draft of air, using either the large forced or induced draft fans, through the catalyst during off line and wash conditions. This provided limited protection from water condensation due to the turbulent flow and carryover of fluid through the catalyst as well as the air being too cool to prevent condensation.
Still other systems used a burner or other heat producing source in the duct to continually provide warm flue gas/air to the catalyst during off line conditions. The major disadvantages of the system were the increased safety risk to plant personnel, extensive capital and operating costs associated with such a system, and the difficulty of having flue gas contaminants in the system during the outage since these contaminants cannot be removed when the system is shut down.
Yet another system reduced the moisture content of the air surrounding the catalyst by isolating the catalyst system and recirculating air through a dehumidifier, which condenses the moisture out of the air. The major disadvantage of this system involved the disposal of condensate and increased maintenance, operating, and capital expense.
In view of the foregoing it is seen that an efficient system for protecting the catalyst during boiler outages was needed which would be cost and operationally effective and not subject the catalyst bed to flue contaminants.